Altera Nios II User Manual page 241

Chapter 8: Instruction Set Reference
Instruction Set Reference
ldbu / ldbuio
Operation:
Assembler Syntax:
Example:
Description:
Usage:
Exceptions:
Instruction Type:
Instruction Fields:
31 30 29 28 27 26 25
A
31 30 29 28 27 26 25
A
February 2014 Altera Corporation
load unsigned byte from memory or I/O peripheral
rB ← 0x000000 : Mem8[rA + σ (IMM16)]
ldbu rB, byte_offset(rA)
ldbuio rB, byte_offset(rA)
ldbu r6, 100(r5)
Computes the effective byte address specified by the sum of rA and the instruction's signed
16-bit immediate value. Loads register rB with the desired memory byte, zero extending the
8-bit value to 32 bits.
In processors with a data cache, this instruction may retrieve the desired data from the cache
instead of from memory. Use the ldbuio instruction for peripheral I/O. In processors with a
data cache, ldbuio bypasses the cache and is guaranteed to generate an Avalon-MM data
transfer. In processors without a data cache, ldbuio acts like ldbu.
For more information on data cache, refer to the
the Nios II Software Developer's Handbook.
Supervisor-only data address
Misaligned data address
TLB permission violation (read)
Fast TLB miss (data)
Double TLB miss (data)
MPU region violation (data)
I
A = Register index of operand rA
B = Register index of operand rB
IMM16 = 16-bit signed immediate value
24 23 22 21 20 19 18 17
B
Instruction format for ldbu
24 23 22 21 20 19 18 17
B
Instruction format for ldbuio
Cache and Tightly Coupled Memory
16 15 14 13 12 11 10 9
IMM16
16 15 14 13 12 11 10 9
IMM16
chapter of
8 7 6 5 4 3 2 1
0x03
8 7 6 5 4 3 2 1
0x23
Nios II Processor Reference Handbook
8–61
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